If you have made a payment to a supplier using a voucher on a booking and you receive a refund in full or part, it is best to process the reversal entry against the same voucher used so that the values are reflected against the booking.

To do this raise a negative invoice for the amount of the refund.  When selecting the voucher you need to enter the negative amount in the This Invoice field.  Please note that if the voucher was closed originally you will get a message about the voucher being closed but you can proceed with the refund.

When you have the invoice processed you can then process a negative cheque against the invoice if you have received the refund, or process the credit with the next payment to the supplier.

After you have processed the refund you can then change the value of the service line in the booking. If you have the correct permission you can change a closed voucher to the value needed and so the booking will reflect the correct profit.

Some additional reference documentation can be found: Cancelling a Cheque Payment